The present invention relates to a high-strength non-woven fabric constituted by fine fibers and a method of producing it. The present invention also relates to a battery separator constituted by such a high-strength non-woven fabric, which shows a small electric resistance.
There are various batteries having different output voltages depending upon the combinations of metals and active materials used for both electrodes, electrolytes, etc.
Recently, attempts have been made to make batteries, particularly those using aqueous electrolytic solutions with higher performance, longer service lives and higher capacities. In connection with this tendency, battery separators are required to have higher capability of retaining an electrolytic solution and excellent capability of preventing electrode active materials particles from penetrating therethrough, with smaller thickness.
As such battery separators having various properties, non-woven fabrics made of thermoplastic resins have been attracting attention.
Such non-woven fabrics are usually produced by a dry method, a wet method and a spun bonding method, etc. However, all of the above methods fail to produce non-woven fabrics consituted by extremely fine fibers which can satisfy recent demands. Also, since these non-woven fabrics do not have sufficiently small port diameters, they do not have good capability of preventing electrode active materials particles from penetrating therethrough. In addition, it is not easy to produce extremely thin non-woven fabrics by these methods.
In these circumstances, attention has been paid to a non-woven fabric produced by a so-called "melt blowing method," in which a molten thermoplastic resin is ejected through a lot of orifices of a die, etc., and drawn by blowing a high-temperature, high-velocity air along the extruded resin, and the resulting fine fibers are accumulated to form webs. The melt-blown non-woven fabrics are superior to the non-woven fabrics produced by the other methods in a fiber diameter and hand. However, the melt-blown non-woven fabrics are poor in mechanical strength because their fibers are extremely fine.
To obviate the problems of poor mechanical strength, the melt-blown non-woven fabrics are conventionally laminated with non-woven fabrics produced by other methods. However, this means deteriorates the hand, gas permeability, etc. which are characteristics of the melt-blown non-woven fabrics, and increases the basis weight of the resulting non-woven fabrics.
Japanese Patent Publication No. 64-2701 discloses a non-woven fabric having improved strength and flexing resistance, which is impregnated with a binder composition comprising (A) a diene copolymer and (B) a water-soluble polyester resin in a weight ratio (A)/(B) of 95:5-50:50 on a solid basis. However, the non-woven fabric impregnated with this binder still fails to show sufficiently high strength. The effect of this binder composition on improving strength, etc. is high for non-woven fabrics made of polyesters, but low for those made of other materials.
With respect to the melt-blown non-woven fabrics, they are used for artificial leathers, etc. because of their small fiber diameters. Needle punching is conducted to increase the strength of the non-woven fabrics. However, this method only slightly succeeds in increasing the strength of the non-woven fabrics, and provides such problems as the increase of production costs and larger pores.
Japanese Patent Publication No. 60-37230 discloses a fiber structure article for artificial leathers having excellent strength and hand, which is constituted by a fiber assembly composed of three-dimensionally entangled monofilaments derived from extremely fine melt-blown fibers having an average diameter of 0.1-5.0 .mu.m, and a padding cloth constituted by a woven fabric embedded in the fiber assembly, part of the fibers in the fiber assembly being entangled with the fibers constituting the padding cloth at an entanglement strength of at least 50 g, a basis weight ratio of the fiber assembly to the padding cloth being 1.5 or more, the entangled fibers having gaps filled with a rubbery elastic polymer, and extremely fine fibers being fluffing on the surface of the fiber structure article. However, since this fiber structure article essentially has a three-layer structure, it has an extremely large basis weight. It also has a relatively large thickness, failing to satisfy the diversified recent demands.
As a result of research, the inventors have found that the non-woven fabrics for battery separators should have:
(1) Sufficient insulating properties when inserted between both electrodes; PA1 (2) Good capability of preventing electrode active materials particles from penetrating therethrough; and PA1 (3) Small electric resistance (high capability of permitting electrolyte ions to pass therethrough).
It has also been found that to improve the above basic properties, in addition to the fact that the non-woven fabric materials per se should have excellent insulating property, the fiber diameter should be smaller than a certain level, and the non-woven fabric itself has high strength, which enables it to have a smaller thickness.